As you said above, Al Ahli actually did not experience a decline in performance, but Al Ahli previous defeat when facing Al Fateh could have been predicted before the match started. Regarding the Al Nassr match that will face Al Ahli, it is actually too early because currently Ronaldo is playing for his country and it is uncertain whether he will appear in the match against Al Ahli later. If in the upcoming match Ronaldo has to be absent for a short rest, Al Ahli still has a chance to hold a draw. But if Ronaldo still appears in the match, it would be better to choose to bet on Al Nassr win -1 with higher odds of around @2+.
Al Nassr should win the game with or without Ronaldo presence because they have being defeating Al Ahli Saudi for the past games that they have played. Ronaldo being fit for the match shouldn't make him to start because he needs to rest after playing during the international break and most of the players in the league didn't have the time play he is having and also considering his age. He shouldn't be over used before he gets burnt out and can't perform again. Both clubs don't have a clean record looking at their last played games but since Ronaldo influences is at Al nassr, they should win or the game ends in a draw for both team to share the points.
Actually, the match between Al Nassr and Al Ahli is not an easy match because basically Al Ahli can make Al Nassr overwhelmed and make it difficult to win, I agree that with the absence of several core players like Ronaldo, there might be chance for Al Ahli to hold draw and share points.
But if Ronaldo is fit, maybe he will still be able to play in this match even, we need to see later when the match started.
Moreover, Ronaldo is very different player from most other top players, he really takes care of his body to be able to perform well, so far Ronaldo has never had any problems with his energy and health even though he has to have more match schedules.